So far, a radio communication network, designed to conduct the interchange of data among a plurality of radio communication terminals, has been in existence. Radio communication methods for use in this radio communication network are roughly classified into a centralized control type communication method in which a radio communication terminal (which will hereinafter be referred to as a radio communication terminal accommodating apparatus), designed to carry out communication management, exists for giving communication rights to each of radio communication terminals and a distributed control type communication method in which no radio communication terminal accommodating apparatus for communication management among other radio communication terminals exists and each radio communication terminal carries out the confirmation of communication situations (carrier sense) in a radio transmission medium (hereinafter referred to as a transmission medium) for preventing a loss stemming from the occurrence of transmitted data collision so that access is made to the transmission medium when the transmission medium is in a vacant state.
In the case of the distributed control type communication method, each radio communication terminal carries out the carrier sense before conducting transmission, and starts transmission after confirming the fact that a transmission medium is in a vacant state (no radio signal exists on the transmission medium). FIG. 13 is an illustration of one example of a conventional distributed control configuration for a radio communication system, and FIG. 14 is a time chart showing one example of communication to be made in the radio communication system shown in FIG. 13. In FIG. 13, a radio communication system including four radio communication terminals 1001 to 1004 is shown as one example. These radio communication terminals 1001 to 1004 are on equal status and, as indicated by arrows, the transmission/reception of data can be made directly between all the radio communication terminals 1001 to 1004.
For example, in a case in which data is transmitted from the radio communication terminal 1001 to the radio communication terminal 1004, the radio communication terminal 1001 first confirms a communication situation of a transmission medium through the use of carrier sense and carries out the data transmission after a predetermined time TD+ random time TR1 elapses from the time at which a signal disappears in the transmission medium. For example, in a period of time 1111 of this transmission operation, a series of operations are conducted as follows. In this connection, during the transmission operation time period 1111, through the use of the carrier sense, each of the other radio communication terminals 1002 and 1003 detects a signal transmitted from the radio communication terminal 1001 to the transmission medium, thereby carrying out the setting so as to inhibit a transmission operation using this transmission medium.
First of all, the radio communication terminal 1001 transmits a transmission request signal indicative of a transmission request to the reception side radio communication terminal 1004. Upon receipt of the transmission request signal, the radio communication terminal 1004 transmits a transmission permission signal indicative of a transmission permission as a reply to the transmission request signal to the transmission side radio communication terminal 1001 and makes a notification on the fact that the preparation for the data reception reaches completion. Upon receipt of this transmission permission signal, the transmission side radio communication terminal 1001 transmits a signal (data signal) on data to the reception side radio communication terminal 1004 while, after the reception of this data, the reception side radio communication terminal 1004 transmits an acknowledgment signal representative of “reception was correctly made” or “reception was not correctly made” to the transmission side radio communication terminal 1001 while, upon receipt of this acknowledgment signal, the transmission side radio communication terminal 1001 terminates a series of operations related to the data transmission from the radio communication terminal 1001 to the radio communication terminal 1004. In this connection, within the transmission operation time period, when the radio communication terminals 1001 to 1004 start the signal transmission, after waiting for a predetermined time Ts (<TD), they conduct the signal transmission. Accordingly, for example, the predetermined time Ts lies between the transmission request signal and the transmission permission signal, between the transmission permission signal and the data signal and between the data signal and the acknowledgment signal.
In addition, in a case in which a plurality of radio communication terminals 1001 to 1004 simultaneously conduct transmissions on the transmission medium, a contention (sometimes referred to equally as collision) occurs for the use of the transmission medium. For avoiding such a contention, at the start of the transmission operation, the radio communication terminals 1001 to 1004 carry out the carrier sense to confirm the fact that the transmission medium is in a vacant state, and each radio communication terminal waits for a random time, thereby avoiding a situation in which, for example, a contention repeatedly occurs between the radio communication terminals so as to make the communication substantially impossible.
For example, in a case in which the radio communication terminal 1002 and the radio communication terminal 1003 try to make data transmissions after the transmission operation time period 1111 for the radio communication terminal 1001, after the transmission operation time period 1111 elapses, the radio communication terminal 1002 tries to conduct a data transmission operation after waiting for a predetermined time TD+ random time TR2, while the radio communication terminal 1003 tries to conduct a data transmission operation after a predetermined time TD+ random time TR3. In this case, when TR2<TR3, the radio communication terminal 1002 first starts the signal transmission and, on the other hand, the radio communication terminal 1003 detects the start of the transmission by the radio communication terminal 1002 through the carrier sense while waiting for its own transmission time, and puts off the transmission. Moreover, the data transmission operation can be conducted after waiting for a predetermined time TD+(TR3−TR2) from when a transmission operation time period 1112 for the radio communication terminal 1002 comes to an end or elapses. That is, after the end of the predetermined time TD+(TR3−TR2 from when a transmission operation time period 1112 comes to an end, a transmission operation time period 1113 starts wherein the radio communication terminal 1003 conducts the data transmission.
In addition, a communication method supporting QoS (Quality of Service), data to be transmitted/received requires, becomes realizable in a manner such that the predetermined time TD or random time TR, serving as a transmission stand-by time, is set at a different value for each type of QoS according to QoS or it is determined through a different algorithm.
On the other hand, in the case of the centralized control type communication method, each radio communication terminal carries out communications with respect to only a radio communication terminal accommodating apparatus and only a radio communication terminal permitted in communication by the radio communication terminal accommodating apparatus can carry out the communications. FIG. 15 is an illustration of one example of a conventional centralized control configuration in a radio communication system, and FIG. 16 is a time chart showing a first example of communication to be conducted in the radio communication system shown in FIG. 15. In FIG. 15, as one example, there is shown a radio communication system including a radio communication terminal accommodating apparatus 1201 and three radio communication terminals 1202 to 1204. As indicated by arrows in FIG. 15, the respective radio communication terminals 1202 to 1204 can make communications with respect to only a radio communication terminal accommodating apparatus 1201.
The radio communication terminal accommodating apparatus 1201 almost periodically transmits an index signal (beacon signal) 1311 for setting a time period for which the radio communication terminal accommodating apparatus 1201 executes centralized control. The radio communication terminal accommodating apparatus 1201 can transmits a signal to a transmission medium after a predetermined time Tp from when signal disappears in the transmission medium, and it can acquire access to the transmission medium with priority to the predetermined time TD (>Tp) for which the radio communication terminals 1202 to 1204 need to wait.
This index signal 1311 enables all the radio communication terminals 1202 to 1204 to carry out a data transmission operation only when receiving a communication permission from the radio communication terminal accommodating apparatus 1201. When the aforesaid predetermined time starts, the radio communication terminal accommodating apparatus 1201 transmits a polling signal (Poll) 1312 to make a notification for selecting a desired radio communication terminal (for example, the radio communication terminal 1202) as a communication partner. At this time, a data signal 1313 to be transmitted can also be transmitted in succession. Upon receipt of the polling signal 1312+ the data signal 1313, the radio communication terminal 1202 transmits a polling reception acknowledgment signal (Poll acknowledgment) 1314 for making a notification on the reception completion after a predetermined time Ts (<Tp). In this connection, the radio communication terminal 1202 can also transmit a data signal 1315 subsequently to the polling reception acknowledgment signal 1314.
After the completion of the transmission by the radio communication terminal 1202, the radio communication terminal accommodating apparatus 1201 can also produce and transmit an acknowledgment signal 1316 for the notification on the completion of reception of the data signal 1315+a polling signal (Poll) 1317 to a different radio communication terminal (for example, the radio communication terminal 1203) after waiting for the predetermined time Ts. In a case in which the radio communication terminal 1203 does not return the polling reception acknowledgment signal 1314 to the radio communication terminal accommodating apparatus 1201 for some reason, the radio communication terminal accommodating apparatus 1201 can also transmit a polling signal (Poll) 1318 to a different radio communication terminal (for example, the radio communication terminal 1204) after a predetermined time Tp. Moreover, when the radio communication terminal accommodating apparatus 1201 transmits an end signal 1319, this centralized control type communication method comes to an end.
Furthermore, FIG. 17 is a time chart showing a second example of communication to be conducted in the radio communication system shown in FIG. 15. The radio communication terminal accommodating apparatus 1201 can select one of the radio communication terminals 1202 to 1204 after a predetermined time Tp from when signal disappears in the transmission medium to transmit a polling signal (Poll) 1411 thereto. This polling signal 1411 makes the setting so as to inhibit the other radio communication terminals 1202 to 1204 from making communications. A transmission-allowable time period 1412 for which only a radio communication terminal to which a transmission right is given through the polling signal 1411 can make data transmission is set after the elapse of a predetermined time Ts from the completion of the transmission of this polling signal 1411. This transmission-allowable time period 1412 runs on from when the polling signal 1411 is transmitted until a predetermined time period elapses or until an end signal 1319 is transmitted by the radio communication terminal accommodating apparatus 1201.
In this connection, the radio communication terminal accommodating apparatus 1201 can transmit this polling signal 1411 at any time when a predetermined time Tp elapses after an arbitrary signal disappears in the transmission medium, and the predetermined time Tp is set at a time period shorter than the predetermined time TD forming a stand-by time of the radio communication terminals 1202 to 1204. Moreover, it is possible to select this radio communication terminal accommodating apparatus 1201 itself through the polling signal 1411 and, since the communication mode is not limited in the transmission-allowable time period 1412 (that is, it is also possible to transmit a polling signal for again selecting the radio communication terminal accommodating apparatus 1201 itself in the transmission-allowable time period 1412), the radio communication terminal accommodating apparatus 1201 has an extremely strong access right to the transmission medium.
Furthermore, FIG. 18 is an illustrative view showing one example of communication based on a combination of conventional distributed control type and centralized control type communication methods. As shown in FIG. 18, the aforesaid distributed control type and centralized control type communication methods are used in a state combined with each other. For example, in a radio communication system having the radio communication terminal accommodating apparatus 1201 shown in FIG. 15, the distributed control type communication method and the communication method of the first example of the centralized control type are almost periodically conducted alternately under control of the radio communication terminal accommodating apparatus 1201, and the radio communication terminal accommodating apparatus 1201 can start the communication method of the second example of the centralized control type.
However, the conventional radio communication methods are mainly for the purpose of avoiding the contention between radio communication terminals and the collision between signals and, hence, the overheads (for example, transmission of polling signals for each radio communication terminal) on communications other than data, such as guard times (for example, the aforesaid predetermined times Ts, Tp, TD and others) and control signals, are provided between signals to be transmitted from each radio communication terminal or a radio communication terminal accommodating apparatus to a radio transmission medium. The data communication time becomes shorter as the communication time for the guard times and the overheads becomes longer, thereby leading to a decrease in throughput, which produces a great factor of a decrease in throughput of data communications. In this case, the throughput signifies an average data transmission volume per unit time which is to be correctly transmitted/received through a radio communication medium between transceivers and, usually, it is expressed by an average number of bits [bit/second] correctly transmitted per second.